The extraction of cobalt(II) from acidic leach liquors, which also contain nickel(II), is typically conducted by solvent extraction with a water-immiscible organic solution containing an organophosphorous acid, which extracts cobalt(II) in preference to nickel(II). Typical acidic leach liquors contain between 1 and 130 g/L nickel and between 0.3 and 25 g/L cobalt. U.S. Pat. No. 4,353,883, issued Oct. 12, 1982 to Rickelton et al., discloses a process to separate cobalt and nickel values from such aqueous solutions by contacting the aqueous solution with a water-immiscible solvent phase containing an organophosphinic acid extractant of the general formula R.sub.1 R.sub.2 PO(OX), where R.sub.1 and R.sub.2 are substituted or unsubstituted alkyl, cycloalkyl, alkoxyalkyl, alkylcycloalkyl, aryl, alkylaryl, aralkyl or cycloalkylaryl radicals, and X is either H or a salt forming cationic species. In this step, cobalt forms an organic soluble complex with the extractant that reports to the organic phase, displacing a stoichiometric amount of X that report to the aqueous phase, along with the majority of the nickel values. After extraction, cobalt in the loaded organic phase can be recovered by stripping with a suitable mineral acid to produce a high concentration cobalt product solution. During this step, the organophosphinic acid is converted to its acid form and is suitable for recycle to the solvent extraction unit operation. Before recycling, the organophosphinic acid can be contacted with a suitable base to displace the hydrogen with the corresponding salt forming radical. A similar solvent extraction process is taught in U.S. Pat. No. 4,348,367, issued Sep. 7, 1982, to Rickelton et al. These patents leave many possible problems unsolved, and in particular, do not deal with a problem of double salt formation, discussed more fully below, and which is specifically addressed by the process of the present invention.
In particular, in the Rickelton et al. patents, if X is a salt forming radical, the extraction reaction will lead to a build-up of this salt, such as sodium sulphate or ammonium sulphate, in the aqueous phase. Similarly, if X is H, the extraction of cobalt will lead to an increase in the acid content of the aqueous phase. This liberated acid has to be neutralized with an appropriate base, such as ammonium or sodium hydroxide, to retain the extractive strength of the organic phase extractant, and in so doing, will also result in a build-up of ammonium or sodium sulphate. The above patents do not address potential problems, such as double salt formation due to a salt build-up, nor do they provide a solution to these problems.
Canadian Patent 1,075,474, issued to Inco Limited (inventors Barnes and Truscott), discloses a process in which cobalt values are separated from nickel values by solvent extraction using an organic solvent phase containing a nickel salt of an organophosphoric acid extractant of the formula (RO).sub.2 PO(OH). The nickel salt of the organophosphoric acid may be generated by first converting the organophosphoric acid to its alkali metal or ammonium salt, and then contacting this salt solution in an organic solvent with a mother liquor from a subsequent nickel salt crystallization step, which produces nickel sulphate crystals from the cobalt solvent extraction raffinate. Alternatively, an organic solution of the acid may be contacted with a nickel base such as nickel hydroxide in an aqueous slurry, although this approach is slow and difficult to use. More preferably, the nickel salt of the organophosphoric acid is generated by mixing an organic solution of the organophosphoric acid with an aqueous solution of nickel sulphate and an aqueous alkaline solution such as sodium hydroxide in a single step. Although ammonium hydroxide is listed as an alternative to sodium hydroxide, there is no teaching of how to avoid the production of double salts or metal hydroxides. For instance, in the case of Example 1 of CA 1,075,474, if ammonium hydroxide had been used as a neutralization reagent in place of sodium hydroxide, a raffinate containing 46.7 g/L Ni and 138 g/L (NH.sub.4).sub.2 SO.sub.4, would have been produced, according to our calculations. Such a composition would, in our experience, result in double salt precipitation. The patent only teaches stoichiometric addition of the neutralizing agent, based on the extraction stoichiometry. In this patent, and in the patents issued to Rickelton et al., there is no mention of ammonium sulphate, the neutralization product, which would be produced if ammonium hydroxide were used for neutralization in the cobalt extraction circuit. More particularly, there is no mention of the potential deleterious effect of ammonium sulphate on the solubility of nickel during the extraction of the cobalt. There is only a teaching of minimizing the contamination of the nickel raffinate with sodium or ammonium.
Furthermore, regarding the prior art associated with the conditions at which a nickel loaded organic phase is formed, there is no mention of the role of, nor in fact, any need for, ammonium sulphate in the feed solution to the nickel loading step as now recognized by the inventors of the present invention. In CA 1,075,474, this feed solution to the nickel loading step is stream 17 in both Figures. Nor is there any recognition of a need to maintain a prerequisite ammonia concentration in this feed solution, in combination with the ammonium sulphate, to prevent the precipitation of the nickel as either a double salt or a hydroxide, hereagain as recognized by the inventors of the present invention. Thus, there is no recognition by the prior art that, as recognized by the inventors of the present application, if the process were practiced according to the patent teachings, the ammonium sulphate would have a deleterious effect, as it would promote the precipitation of the nickel as a double salt.
Overall, CA 1,075,474 can be summarized by the following equations, when sodium hydroxide is used for neutralization, with the overbar representing the organic phase and RH representing the organophosphorous acid extractant: EQU Extraction: 2RH+CoSO.sub.4 +2NaOH{character pullout}R.sub.2 +L Co+Na.sub.2 SO.sub.4 +2H.sub.2 O (1) EQU Stripping: R.sub.2 +L Co+H.sub.2 SO.sub.4 {character pullout}2RH+CoSO.sub.4 (2)
In reality, the base used for neutralization must be carefully selected based on the operability of the solvent extraction circuit and in consideration of the other process units, as it will accumulate in the nickel containing raffinate. If, for instance, electrowinning is selected for final recovery of nickel from the solvent extraction raffinate, it would be undesirable to use ammonium hydroxide for neutralization, as ammonium cannot be tolerated during the electrowinning step, and sodium hydroxide will be the preferred neutralization reagent.
If hydrogen reduction is selected for final nickel recovery, sodium hydroxide will be undesirable, as it will lead to sodium contamination of not only the nickel product, but also the ammonium sulphate salt, which is recovered from the barren reduction end solution by crystallization. Therefore, if hydrogen reduction is used, ammonium hydroxide will be the preferred neutralization reagent. However, this reagent has a disadvantage in that the ammonium sulphate produced during the solvent extraction of cobalt, will reduce the solubility of nickel in the raffinate, depending on the relative metals and ammonium sulphate concentrations, the temperature and the solution pH. If the solubility limit is exceeded, nickel will precipitate from solution as a nickel sulphate-ammonium sulphate double salt (NiSO.sub.4.(NH.sub.4).sub.2 SO.sub.4.6H.sub.2 O).
If ammonium hydroxide is substituted in reaction (1) for the neutralization, the amount of ammonium sulphate produced is stoichiometric to the amount of cobalt extracted, i.e., the higher the cobalt concentration in the feed solution, the more ammonium sulphate will be produced. Reduced nickel solubility will, therefore, not present a significant problem if solutions with low total metals (cobalt and nickel) concentrations are treated. However, solutions with high total metals concentrations, typically more than 100 g/L, are usually targeted to reduce the capital and operating costs in a metals refinery. Precipitation of double salts during the treatment of these streams is, therefore, a greater possibility under conditions such as those targeted in the cobalt extraction circuit, especially if the feed solution has a high cobalt and nickel content.
One solution is to dilute the leach solution prior to separation of cobalt and nickel by solvent extraction. However, this will reduce the nickel concentration in the raffinate, requiring larger final recovery equipment, or necessitating an additional nickel recovery step.
The patents issued to Rickelton et al., and CA 1,075,474 both mention the use of a nickel form of the organic extractant, however, no other teaching is provided of how to practice their processes with this extractant in order to avoid the production of double salts.
The solvent extraction process with neutralization, for the specific case of nickel and cobalt separation can be simplified as follows, RH representing the organophosphorous acid extractant and the overbar representing the organic phase: EQU Extraction: 2RH+CoSO.sub.4 {character pullout}R.sub.2 +L Co+H.sub.2 SO.sub.4 (3) EQU Neutralization: H.sub.2 SO.sub.4 +2NH.sub.3.fwdarw.(NH.sub.4).sub.2 SO.sub.4 (4) EQU Combined: 2RH+CoSO.sub.4 +2NH.sub.3 {character pullout}R.sub.2 +L Co+(NH.sub.4).sub.2 SO.sub.4 (5)
Canadian Patent 2,098,638, issued Apr. 21, 1998, to Outokumpu Harjavalta Metals Oy, and U.S. Pat. No. 5,779,997, issued Jul. 14, 1998, and assigned to this same company, disclose a method to prevent the formation of jarosite in the leaching step, and ammonium-based double salts in the solvent extraction step, in a process to separate valuable metals, cobalt and nickel being specific examples, from acidic solutions generated in leaching processes. An organic solvent phase containing an organophosphoric, organophosphonic or organophosphinic extractant is used to achieve cobalt-nickel separation. The patents propose a process whereby the acid form of the organic extractant is first neutralized with an alkaline solution such as ammonium or sodium hydroxide, to convert the organic to the corresponding salt form. In a second step, this organic salt solution is contacted with a preload solution containing an intermediate metal, in this case magnesium, to produce an organic phase loaded with the intermediate metal, and a salt discharge solution from which the salt can be recovered by crystallization. In a further process step, the organic phase loaded with the intermediate metal is contacted with the cobalt and nickel containing solution. In this step, the intermediate metal is displaced from the organic phase to produce a cobalt-loaded organic phase and a nickel raffinate that is enriched in the intermediate metal. As ammonium or sodium double salt or jarosite forming species are not displaced into the nickel containing aqueous raffinate, the likelihood of precipitation during cobalt recovery by solvent extraction is considerably reduced. An additional amount of the intermediate metal is added to barren solution from one of the final nickel recovery steps, hydrogen reduction, to generate the solution for the intermediate metal loading step, while barren solution from the electrowinning circuit is recycled to the leach step.
To the inventors' knowledge, the above outlined method has never been commercially used and has a number of potential difficulties, including:
The intermediate metal used, in this case magnesium, has to be recovered downstream from the solvent extraction circuit, in this specific case, from the nickel reduction end solution or spent electrolyte. PA1 Due to incomplete recovery of the intermediate metal, a make-up is required. PA1 Due to incomplete recovery of the intermediate metal in the loading step, contamination of the final products, in this case ammonium sulphate, cobalt and nickel, by the intermediate metal, in this case magnesium, can be expected. PA1 In the intermediate solvent extraction step, essentially complete extraction of the intermediate metal is required to reduce the make-up requirement of the intermediate metal, and to produce an aqueous discharge solution suitable for the recovery of a pure salt product by crystallization, in this case ammonium sulphate. PA1 A large number of theoretical stages are required to get reasonable extraction of the intermediate metal in the preload step. PA1 a) contacting a portion or all of the water-immiscible organic solution required for cobalt extraction with a nickel-containing ammoniacal solution to produce a nickel-loaded organic phase and a partially nickel-depleted raffinate; and PA1 b) passing the nickel-loaded organic phase to the cobalt extraction circuit for selective cobalt extraction in which cobalt displaces nickel from the organic phase to produce a cobalt-depleted, nickel-enriched, raffinate, and a cobalt-loaded organic phase. PA1 a) contacting the water-immiscible organic solution required for cobalt extraction with a nickel-containing ammoniacal solution in a nickel preload step to produce a nickel-loaded organic phase and a partially nickel-depleted raffinate; PA1 b) contacting the aqueous cobalt-containing nickel sulphate solution in the cobalt extraction circuit with the nickel-loaded organic phase from step (a) to produce a cobalt-depleted, nickel-enriched, raffinate, and a cobalt-loaded organic phase; PA1 c) optionally recycling a portion or all of the cobalt-depleted, nickel-enriched, raffinate from (b) to a solution adjustment step in which ammonia, preferably as ammonium hydroxide, and ammonium sulphate is added to the cobalt-depleted, nickel-enriched, raffinate to produce the nickel-containing ammoniacal solution used in step (a); PA1 d) optionally combining the partially nickel-depleted raffinate from step (a) with the remaining cobalt-depleted, nickel-enriched, raffinate from step (b); PA1 e) recovering cobalt from the cobalt-loaded organic phase from step (b); and PA1 f) recovering nickel from the aqueous raffinates from step (a) and (b), or from the combined aqueous raffinates from step (d).
There is still a need for a viable process to separate nickel and cobalt from acidic sulphate leaching solutions, which accomplishes neutralization in accordance with the above reactions, but avoids the formation of double salts.